he detection of a cosmic wake would nonetheless be "extremely unlikely". He says the amplitude of a wake would have to be just right: too small and we wouldn't see it; too big and it would probably have had severe consequences for our universe's structure. The number of collisions would also have to be "fine-tuned", he says.

"The claim seems to be that we might see one or two wakes in our sky, but why one or two?" he adds. "Why not none or an infinite number? In fact, if bubble collisions were common we would not be alive to discuss the question."

Cosmologist Arjun Berera at the University of Edinburgh, UK, also thinks the idea of a multiverse – and by extension Sigurdson and colleagues' prediction – is speculative. But he notes that a positive detection would be "spectacular". "Such a case would offer suggestive evidence in support of string theory," he says. "On the other hand, no evidence in the CMB data for a collision between two universes would not rule out string theory, it would simply extend the widely held belief in the field that string theory is unfalsifiable."

What matters, what's at the heart of the subject, is whether there exist realms that challenge convention by suggesting that what we've long thought to be the universe is only one component of a far grander, perhaps far stranger, and mostly hidden reality.

In some, the parallel universes are separated from us by enormous stretches of space or time; in others, they're hovering millimeters away; in others still, the very notion of their location proves parochial, devoid of meaning. A similar range of possibility is manifest in the laws governing the parallel universes. In some, the laws are the same as in ours; in others, they appear different but have shared a heritage; in others still, the laws are of a form and structure unlike anything we've ever encountered. It's at once humbling and stirring to imagine just how expansive reality may be.

want to encourage everyone to be more open to the multiverse hypothesis because there is more evidence coming in that is supporting it (don’t get me wrong, there is contrary evidence that must be weighed as well). What is beautiful about this whole situation is that cosmologists and theoretical physicists predicted the multiverse from mathematical equations (and no doubt philosophical presuppositions). If the multiverse hypothesis is true it’s a beautiful discovery because we would have gone from pencil and paper with numbers to actually finding what was predicted by those numbers. We do live in an elegant universe.

The image is based on the initial 15.5 months of data from Planck and is the mission’s first all-sky picture of the oldest light in our Universe, imprinted on the sky when it was just 380 000 years old.

At that time, the young Universe was filled with a hot dense soup of interacting protons, electrons and photons at about 2700ºC. When the protons and electrons joined to form hydrogen atoms, the light was set free. As the Universe has expanded, this light today has been stretched out to microwave wavelengths, equivalent to a temperature of just 2.7 degrees above absolute zero.

This ‘cosmic microwave background’ – CMB – shows tiny temperature fluctuations that correspond to regions of slightly different densities at very early times, representing the seeds of all future structure: the stars and galaxies of today.

According to the standard model of cosmology, the fluctuations arose immediately after the Big Bang and were stretched to cosmologically large scales during a brief period of accelerated expansion known as inflation.

Planck was designed to map these fluctuations across the whole sky with greater resolution and sensitivity than ever before. By analysing the nature and distribution of the seeds in Planck’s CMB image, we can determine the composition and evolution of the Universe from its birth to the present day.

Getting smacked by an entire universe would definitely leave a mark. Specifically, it would leave a disk-like pattern in our universe's cosmic microwave background radiation, and this is what a group of cosmologists from University College London have been looking for. Somewhat incredibly, they managed to find some of these bruises. Four of them. And it's ten times more likely that the four marks are universe collision signatures than that they are anything else that we know of.

Multiverse theory, if true, could explain a number of things including what caused the big bang that created the universe, what existed before the big bang, and why it is the universe appears precisely tuned to host human life, at least on planet Earth.

aura Mersini-Houghton, theoretical physicist at the University of North Carolina at Chapel Hill, and Richard Holman, professor at Carnegie Mellon University, both theorized in 2005 these anomalies could be caused by the pull of other universes outside our own.

Scientific American's John Horgan wrote that he used to get fired up over the idea that our universe was just one of many making up a grander "multiverse." But not anymore:

"Now, multiverse theories strike me as not only unscientific but also immoral, for two basic reasons: First, at a time when we desperately need science to help us solve our problems, it's irresponsible for scientists as prominent as Greene to show such a blithe disregard for basic standards of evidence. Second, like religious visions of paradise, multiverses represent an escapist distraction from our world."

"My own moral concerns about the multiverse have more to do with worry that pseudo-science is being heavily promoted to the public, leading to the danger that it will ultimately take over from science, first in the field of fundamental physics, then perhaps spreading to others."

As a string theorist, Greene is used to such criticism. Like parallel universes, the idea that matter's fundamental building blocks are tiny vibrating strings or multidimensional membranes has often been knocked as unprovable, unverifiable, unfalsifiable speculation. Lawrence Krauss, a theoretical physicist at Arizona State University, is fond of saying that string theory's vision of a "theory of everything" is actually a "theory of anything" that turns out being a "theory of nothing."

"That's provocative nonsense," Greene told me last week. Theorists are not just pulling this stuff out of thin air, he said. Rather, they're being led to seemingly wild conclusions while working within what he called "the tight straitjacket of mathematics."

That does not mean that there are. It does mean, however, that there's a compelling enough reason to take these ideas seriously, develop them further, and try to make contact with observation and experiment.

As we have developed mathematical tools to fill in that gap, to really understand what happened at the beginning, the math has indicated that the big bang may not have been a unique event. There may have been, and may continue to be, many big bangs — each of which gives rise to its own expanding universe, our universe being but one among many. In that sense, we are part of a multiverse.

This idea is controversial for good reason. It is at the cutting edge — not only the cutting edge of science, but also the edge of the kinds of ideas that we want to embrace in science. That's what makes it exciting.

over the past 500 years, each new cosmological discovery has removed us from being in a privileged position in the universe. First, we realized that the sun, not the earth, was the center of the solar system; then we discovered that our sun is just an ordinary star in the Milky Way, and that our galaxy is just an ordinary galaxy among about 80 billion or so that are visible. Greene takes this further: what we call our universe is only one piece of an unbelievably vast multiverse.

there are very good reasons to think that the big bang created an infinite universe. If matter is more or less evenly distributed through the whole thing, then there must be other pieces that look just like ours, other islands in the cosmic sea, each with a radius of 14 billion light years, that are simply beyond our limited horizon. These islands make up the patchwork of a quilted multiverse.

The only explanation which anyone could come up with is that the particles don't just exist in our Universe. They flit into existence in other universes, too and there are an infinite number of these parallel universes, all of them slightly different.

The weakness of gravity could at last be explained, but only by introducing the idea of a parallel universe. Randall's idea opened a Pandora's Box. Now suddenly physicists all over the world piled into the eleventh dimension trying to solve age-old problems and every time it seemed the perfect explanation was another parallel universe

Was there a beginning? Did time continue before the Big Bang? Where did the Universe come from?

NARRATOR: Above all, they were still trying to solve the biggest problem of all: what caused the very start of the Big Bang, the singularity?

NEIL TUROK: Nobody has a solution for the singularity problem other than essentially by hand starting the Universe at a certain time and saying let's go from there and let's not worry about what happened before and that's very unsatisfactory. This is the deepest problem in cosmology.

M Theory may really be able to explain everything in the Universe, but the victory will be bittersweet, for at the end of its long quest, science has discovered that the Universe it's thought to explain may be nothing special. It is nothing more than one of an infinite number of membranes, just one of the many universes which make up the multiverse.

The latest understanding of the multiverse is that there could be an infinite number of universes each with a different law of physics. Big Bangs probably take place all the time. Our Universe co-exists with other membranes, other universes which are also in the process of expansion. Our Universe could be just one bubble floating in an ocean of other bubbles.

The multiverse, if it exists, may have sprung out of a chaotic fluctuation of empty space.

Several "bubble" universes similar to our own, but perhaps with slightly different physical laws, would have appeared at about the same time and bumped against each other before diffusing across the multiverse.

The algorithm has, so far, found 15 interesting features. Four of these looked especially promising, but statistical analyses suggest chance is the best explanation for the features, according to co-author Johnson.

the multiverse approach has its roots in the world of theoretical physics. Hugh Everett III's 1950s Many-Worlds theory argued that the universe we know is but one of infinite parallel universes, each different from the last.

there are regions of the Universe where it inflated in the past, that false-vacuum energy got turned into radiation and matter, and those parts of the Universe had a history very much like our own. But in between those regions, there are other parts that keep on inflating, and so on, and so on, and so on…

ew calculations by quantum physicists now suggest that the stargate idea might actually be the better theory. According to the startling new results, black holes do not culminate in a singularity. Rather, they represent "portals to other universes,

Interestingly, this same principle can be applied to the Big Bang. According to conventional theory, the Big Bang started with a singularity. But if time is rewound according to LQG instead, the universe does not begin with a singularity. Rather, it collapses into a sort of tunnel, which leads into another, older universe. This has been used as evidence for one of the Big Bang's competing theories: the Big Bounce.

Today, scientists have observed strange new motion at the very limits of the known universe - kind of where you'd expect to find new things, but they still didn't expect this. A huge swath of galactic clusters seem to be heading to a cosmic hotspot and nobody knows why.

most cosmological models have things moving in all directions equally at the extreme edges of the universe. Something that could make things aim for a specific spot on such a massive scale hasn't been imagined before. The scientists are keeping to the proven astrophysical strategy of calling anything they don't understand "dark", terming the odd motion a "dark flow".

A black hole can't explain the observations - objects would accelerate into the hole, while the NASA scientists see constant motion over a vast expanse of a billion light-years. You have no idea how big that is. This is giant on a scale where it's not just that we can't see what's doing it; it's that the entire makeup of the universe as we understand it can't be right if this is happening.

One explanation that's already been offered is that our universe underwent a period of hyper-inflation early in its existence, and everything we think of as the vast and infinite universe is actually a small corner under the sofa of the real expanse of reality. Which would be an amazing, if humbling, discovery.

That would mean that black holes can serve as portals to other universes. While other theories, not to mention some works of science fiction, have suggested this, the trouble was that nothing could pass through the portal because of the singularity. The removal of the singularity is unlikely to be of immediate practical use, but it could help with at least one of the paradoxes surrounding black holes, the information loss problem.

A black hole soaks up information along with the matter it swallows, but black holes are also supposed to evaporate over time. That would cause the information to disappear forever, defying quantum theory. But if a black hole has no singularity, then the information needn't be lost – it may just tunnel its way through to another universe. "Information doesn't disappear, it leaks out,"